Parkinson's disease is a degenerative condition characterized by reduced concentration of the neurotransmitter dopamine in the brain. Levodopa (L-dopa or L-3,4-dihydroxyphenylalanine) is an immediate metabolic precursor of dopamine that, unlike dopamine, is able to cross the blood-brain barrier and is most commonly used for restoring the dopamine concentration in the brain. For the past 40 years, levodopa has remained the most effective therapy for the treatment of Parkinson's disease.
However, levodopa has a short half-life in plasma that, even under best common current standard of care, results in pulsatile dopaminergic stimulation. Long-term therapy is therefore complicated by motor fluctuations and dyskinesia that can represent a source of significant disability for some patients. A therapeutic strategy that could ultimately deliver levodopa/dopamine to the brain in a more continuous and physiologic manner would provide the benefits of standard levodopa with reduced motor complications and is much needed by patients suffering from Parkinson's disease and other neurological or movement disorders (Olanow C W; Mov. Dis. 2008, 23 (Suppl. 3):S613-S622). Sustained-release oral levodopa formulations have been developed, but, at best, such preparations have been found to be no more efficacious than standard tablets. Continuous administration of levodopa by intraduodenal administration or infusion has also been attempted by using ambulatory pumps or patches. Such treatments, especially intraduodenal, are extremely invasive and inconvenient.
The metabolic transformation of levodopa to dopamine is catalyzed by the aromatic L-amino acid decarboxylase enzyme, a ubiquitous enzyme with particularly high concentrations in the intestinal mucosa, liver, brain, and brain capillaries. Due to the possibility of extracerebral metabolism of levodopa, it is necessary to administer large doses of levodopa leading to high extracerebral concentrations of dopamine that cause nausea in some patients. Therefore, levodopa is usually administered concurrently with oral administration of a dopa decarboxylase inhibitor, such as carbidopa or benserazide, which reduces by 60-80% the levodopa dose required for a clinical response, and thus prevents certain of its side effects by inhibiting the conversion of levodopa to dopamine outside the brain.
Various oral formulations together with inhibitors of enzymes associated with the metabolic degradation of levodopa are well known, for example, decarboxylase inhibitors such as carbidopa and benserazide, monoamone oxidase (MAO)-A or MAO-B inhibitors such as moclobemide, rasagiline, selegiline, and safinamide, and catechol-O-methyl transferase (COMT) inhibitors such as tolcapone and entacapone. Currently available oral drugs include SINEMET® and SINEMET® CR sustained-release tablets that include carbidopa or levodopa; MADOPAR® tablets containing levodopa and benserazide; and STALEVO® tablets containing carbidopa, entacapone, and levodopa.
Carbidopa is a non-competitive inhibitor of DOPA decarboxylase. When mixed with levodopa, carbidopa inhibits the peripheral conversion of levodopa to dopamine. This results in increased levodopa available for transport to the CNS. Carbidopa also inhibits the metabolism of levodopa in the GI tract, thus, increasing levodopa bioavailability. It is used in Parkinson's disease to reduce the peripheral effect of dopamine. The loss of the hydrazine functional group represents the major metabolic pathway for carbidopa.
Hydrazine (N2H4), or its salts, are used in the production of pharmaceutical products. It has been the cause of severe adverse effects on the central nervous system, liver, and kidneys. In addition to these effects, experimental animals have also shown the following symptoms: loss of body weight, anaemia, hypoglycaemia, fatty degeneration of the liver, and convulsions. Hydrazine has also been shown to cause DNA damage, gene mutations, and chromosome aberrations (Environmental health criteria No. 68 hydrazine (1987)) and has induced tumor growth in mice, hamsters, and rats after oral, intraperitoneal, and inhalation administration (MacEwan J D, Vernot E H, Haun C C, et al. (1981)). Hydrazine and its salts are used in the pharmaceutical industry as an intermediate to produce drugs with different therapeutic effects including decarboxylase inhibitors, antihypertensives, and antibacterials. Since hydrazine is toxic and thought to be a possible human carcinogen, its presence is limited in some of these drug substances in the monographs of the European Pharmacopoeia (Ph. Eur.).
Accordingly, there is an ongoing and urgent need for liquid formulations and compositions that can achieve continuous dopaminergic stimulation to treat movement disorders such as Parkinson's disease more effectively containing a safe and tolerable amount of hydrazine.